pkgbase: Create a FreeBSD-mtree package

And put the mtree binary and files in it.
Useful to create small mfsroot using /etc/rc.d/var without
having to install FreeBSD-utilities.

MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33442